cytochrome b/c1

**Location and structure:** Complex III is embedded in the inner mitochondrial membrane and functions as a dimer. Each monomer contains 3 catalytic subunits: cytochrome b (with heme bL and bH, the only subunit encoded by mitochondrial DNA), cytochrome c1 (with heme c1), and the Rieske iron-sulfur protein (2Fe-2S cluster). Other subunits are nuclear-encoded.

**Function:** It oxidizes ubiquinol (CoQH2) and reduces cytochrome c, a mobile 1-electron carrier in the intermembrane space. It does this through the **Q cycle**: - Ubiquinol is oxidized at the Qo site (the P side, facing the intermembrane space). One electron goes to the Rieske Fe-S center, then to cytochrome c1, then to cytochrome c. The other electron goes through heme bL and bH to the Qi site (the N side, facing the matrix). Two protons are released into the intermembrane space. - At the Qi site, ubiquinone is reduced to a semiquinone, and then to ubiquinol after a second round, which takes up 2 protons from the matrix. - Net result per 2 ubiquinol oxidized: 4 H+ released into the intermembrane space, 2 H+ consumed from the matrix, 2 cytochrome c reduced, and 1 ubiquinol regenerated. The net is 2 H+ pumped per electron pair passing through to cytochrome c.

**Electron flow:** NADH → Complex I → CoQ → **Complex III** → cytochrome c → Complex IV → O2. Succinate → Complex II → CoQ → Complex III (this bypasses Complex I).

**Inhibitors (high-yield):** - **Antimycin A** binds the Qi site. - **Myxothiazol and stigmatellin** bind the Qo site. - Inhibition blocks electron flow and proton pumping. The chain upstream becomes reduced, ATP synthesis falls, and the cell shifts to anaerobic glycolysis, which raises lactate. - Cyanide, CO, and azide inhibit Complex IV, not Complex III. Don't confuse them. - **Atovaquone** (antimalarial/antipneumocystis) selectively inhibits the parasite's cytochrome bc1 (Qo site). It is used for malaria prophylaxis (with proguanil), for *Pneumocystis jirovecii*, and for babesiosis (with azithromycin).

**Clinical correlations:** - Mutations in **MT-CYB** (the mitochondrial cytochrome b gene) cause exercise intolerance, myopathy, and rarely cardiomyopathy or multisystem disease. Because the gene is mitochondrial, inheritance is maternal (mitochondrial inheritance, heteroplasmy). - Complex III deficiency can cause lactic acidosis and ragged red fibers. Nuclear genes such as BCS1L (an assembly factor) can also cause deficiency, with autosomal recessive inheritance. This produces GRACILE syndrome and some Björnstad syndrome cases. - Complex III is a major site of reactive oxygen species (ROS) production. Semiquinone at Qo can leak electrons to O2 to form superoxide. - Electron transport chain inhibitors differ from uncouplers (2,4-DNP, aspirin overdose, thermogenin/UCP1). Uncouplers dissipate the proton gradient while electron transport continues, so O2 consumption increases and heat is produced. Inhibitors of the chain decrease O2 consumption.

**Board pearl:** Cytochrome c release from the intermembrane space into the cytosol triggers apoptosis through the intrinsic pathway (Apaf-1 → apoptosome → caspase-9). Cytochrome c is the carrier between Complexes III and IV.